Reciprocating energy-conversion devices, such as ORCs using reciprocating expanders or Thermofluidic Oscillators (TFOs), are promising technologies for the conversion of low- or medium-grade heat from waste heat, solar, or geothermal sources at power outputs below 100 kW. In this thesis, three topics associated with reciprocating machines are investigated. Firstly, a novel two-phase TFO engine concept, named ERPE, is modelled using a linear dynamic approach. Secondly, the use of working-fluid mixtures in ORCs is examined. Thirdly, unsteady heat-transfer losses associated with reciprocating motion are investigated. The ERPE is a theoretical TFO concept especially well-suited as a prime-mover in combined heat and power applications with power outputs below 10 kW. Based on thermal/fluid-electrical analogies, the theoretical engine is conceptualized in the electrical analogy domain as a linearized closed-loop active circuit model. By comparing model calculations with measurements of an existing prototype that is similar to the concept, it has been shown that the linear model provides realistic results. The effects of liquid inertia, viscous drag, hydrostatic pressure, vapour compressibility, and two-phase heat transfer in the various engine components/compartments are examined. Measures for improving engine performance are provided which demonstrate how this engine concept can outperform competing TFO solutions. The methodological approach implemented in this study can be used to guide the early-stage design and verification of these engines, while offering important guidelines for optimizing performance. For the second topic of the thesis, the use of working fluid mixtures in ORCs and their influence on thermodynamic performance is investigated. Mixtures can potentially reduce exergy losses due to their non-isothermal phase-change behaviour. The efficiency, power output, and cost are calculated as a function of working-fluid mixture composition in a mathematical model of a sub-critical, non-regenerative ORC. A new version of the statistical associating fluid theory equation of state is used to predict unavailable thermodynamic property data required for the ORC model. When unlimited quantities of cooling water are used, cycles with the lighter pure working fluid exhibit the highest efficiencies and power outputs and lowest costs. Only at low evaporation pressures do the investigated mixtures perform better than pure fluids. When the quantity of cooling water is constrained by the application, overall performance deteriorates and mixtures emerge as the optimal working fluids. A general conclusion from this work is that the choice of the optimum working fluid (pure or mixture) depends in a non-trivial and occasionally counter-intuitive fashion on the particular external heat source and heat sink conditions. The third topic revolves around unsteady heat-transfer losses which occur in reciprocating energy-conversion devices. These exergy losses occur even if the overall time-averaged heat transfer between the gas and the cylinder wall are zero. The specific aim is to compare the behaviour of real gases (such as those used in ORCs) to ideal gases by simulating an oscillating gas-spring in CFD . Results show that unsteady heat-transfer losses are not negligible as they can reach up to 30 % of the work required to compress the gas. Further results indicate that simple mono- and diatomic gases exhibit negligible differences between ideal and real-gas models. However, when considering heavier (organic) molecules, the ideal gas-models overestimate the pressure by as much as 20 % and underestimate the unsteady heat-transfer losses by 25 %. The increase in loss is caused by different compressibility factors of the gas during compression and expansion.